1. Field of the Invention
The present invention relates to a diffractive optical element used for various kind of optical instruments.
2. Related Background Art
A diffractive optical element for diffracting light emitted from a light source such as a laser diode or the like and splitting it into a main beam and a sub-beam has conventionally been attached to an optical pickup incorporated in a CD/DVD reproducing apparatus or the like. FIG. 5 shows a conventional diffractive optical element. The diffractive optical element 101 shown in FIG. 5 is made by injection-molding a resin such as acrylics and has an outside diameter of about 4 to 5 mm. As depicted in this drawing, the center part of the diffractive optical element 101 is formed with an optically functioning part 103 having a diffraction grating 102. Around the optically functioning part 103, a flange part 104 is formed. The outer circumferential face 105 of the flange part 104 is formed like a cylindrical surface.
The diffractive optical element 101 is attached to a holder section 110 formed in an optical pickup or the like. In this case, the diffractive optical element 101 is inserted into an attachment hole 110a incorporated in the holder section 110 and is secured thereto with a coned disc spring or the like in a state where the diffraction grating 102 is oriented to a predetermined direction. When attaching the diffractive optical element 101 to the holder section 110, it is necessary to prevent the diffractive optical element 101 from shifting its position. Therefore, the gap between the outer circumferential face 105 of the flange part 104 and the inner circumferential face of the attachment hole 110a of the holder section 110 should be as small as possible.